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The definition of a kilogram has been the same since 1889. Now scientists have changed it.

Members of the French Academy of Sciences hold a conference to discuss atomic energy in 1945. The group includes (from left) Frederic Joliot-Curie (1900 - 1958), the Duke de Broglie, Paul Langevin (1872 - 1946) and Prince de Broglie. The French Academy of Sciences has decided to redefine the standard for the kilogram. (Photo by Keystone/Hulton Archive/Getty Images)

Scientists gathered in France have decided to redefine the definition of a kilogram.

What does this mean?

“The vote is a unanimous yes, I hope that such votes will be possible for many other issues in the world,” professor Sebastien Candel, president of the French Academy of Sciences, declared.

But why change the kilogram at all?

Since 1889, the kilogram has been defined as the exact weight of an actual piece of metal dubbed “Le Grand K” sitting in the International Bureau of Weights and Measures in Sèvres, France.

While not every metric bathroom scale may have been set to correspond precisely to this cylinder of platinum-iridium alloy, more precise measurements for industries like agriculture, medicine, and science have all used this standard to come to an agreed upon measurement when dealing with super-specific amounts. U.S. President Benjamin Harrison was given a copy of Le Grand K the same year.

But scientists started to notice a problem with Le Grand K. Even though it was kept in a secure location and protected by glass bell jars, the metal had been slowly losing mass.

By 2018, the gold standard for the kilogram had lost 50 micrograms and become 99.999995 percent of its original mass. Such a small difference would not affect most people, but for industries that rely on very exact measurements, this difference would require constant adjustment to maintain accuracy.

So the scientists gathered Friday at the General Conference on Weights and Measures decided to retire Le Grand K and move on to something that couldn't be changed: the mathematical concept known as Planck's constant.

Planck's constant is one of the smallest measures possible (6.62607004 × 10-34 m2 kg / s), and, more importantly, it never changes.

What else?

The kilogram is not the first measurement to be redefined.

When it was first defined back in 1791, the French Academy of Sciences declared that a meter was 1/10 millionth of the distance from the North Pole to the equator of a line running through Paris.

In 1889, the same year Le Grand K was cast, the International Bureau of Weights and Measures created a bar of platinum and iridium alloy with two lines on it that were exactly one meter apart.

In 1960, the wavelengths of the spectrum in a krypton-86 atom were used to define a meter, and in 1983, scientists finally decided to settle on defining it as the distance light could travel in a vacuum in 1/299,792,458 second.

One last thing…
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